Analog amplifiers having a variable switched gain have been known in the art for amplifying analog signals at either a continuously variable gain or a discretely variable gain. One type of variable gain amplifier is that utilizing an operational amplifier wherein the gain of the amplifier is determined by the ratio of a feedback resistance leg to a series resistance leg. In these types of amplifier circuits, variable type resistance devices can be utilized or, in programmable type devices, discrete components such as capacitors and resistors are utilized in conjunction with various switches.
One application of a programmable switched gain amplifier is as an analog interface for an analog to digital (A/D) converter. This type of application requires an amplifier that has a continuous analog output with a very low distortion. Typically, the input signal is comprised of an A/C signal ranging from DC to 4 Khz.
Some disadvantages with prior devices utilizing switching of discrete components is recognized in the distortion characteristics. For very small signals such as those incurred in high resolution A/D converters, any distortion can be detrimental. This distortion is normally due to the series resistance of the switching devices that are utilized to switch the components. This series resistance affects the harmonic output of the amplifier due to the series current flowing through both the input and feedback legs of the amplifier. There therefore exists a need for a programmable switched gain amplifier having a continuous analog output and low harmonic distortion.